The mosquito melanization response in defense agaisnt the Plasmodium vivax infection

Background: Mosquito immunity studies have focused mainly on characterizing immune effector mechanisms elicited against parasites, the melanization is an immediate immune response in arthropods and requires activation of pro-phenoloxidase (PPO) by a proteolytic cascade converting it to phenoloxidase (PO) leading to melanin production. In this work we used the malaria vector Anopheles aquasalis as a model to investigate the role of melanization in mosquito defense against Plasmodium vivax and determine if this mechanism represents a general response to malaria sporozoites by the mosquito vector and its impact on the infection outcome. Methods: Adult An. aquasalis females were obtained from a colony at Fundação de Medicina Tropical Dr. Heitor Vieira Dourado (FMT– HVD Manaus, Amazonas, Brazil. Mosquitoes were blood-fed using a membrane-feeding device containing infected blood from malarial patients. To evaluate the role of melanization on P. vivax infection outcome, infected mosquitoes were injected with a PO inhibitor (N-Phenylthiourea - PTU) at day 7 post-infection (p.i.). Infection was followed on treated and non-treated mosquitoes from D7pi, when sporozoites are still developing inside the oocysts, up to D14pi. Salivary gland invasion was evaluated by qPCR. Results: Melanized oocysts were observed from D11pi onwards. The rate of melanized oocysts were lower in PTU-injected mosquitoes (27.7% - 44%) compared to the control group (52% - 74%). Conclusions: Melanization seems to be necessary to help restrain infection and sporozoite transmission.